Digital printing as a technique is known and widely used in colour printing, copying machines and printers, among others. EP Patent Application 629930 describes digital printing techniques producing multi-colour print on one or both sides of a moving paper web. The different printing colour shades are produced at consecutive synchronised printing stations placed along the web path. Each station comprises a rotating drum with a charger disposed at its periphery for generating a regular electric charge on the surface of the drum. At the drum periphery, the charger is followed by a print head, such as a laser scanner, which generates a latent image on the surface of the drum by selective modification of the charge of the drum surface, the latent image being subsequently developed at a development station, where printing colour particles charged with opposite signs are brought to locations on the drum surface corresponding to the image. After this, the drum surface is contacted with a paper web guided to pass by laterally in order to transfer image-forming printing ink particles to the web surface. To this end, a corona transfer device has been mounted at the tangential point between the drum and the web, at the opposite side of the web, the electric current led over the corona transfer device generating an electric field, which attracts electrically charged printing ink particles from the drum surface to the paper web surface. In the immediate vicinity of the corona transfer device, an alternating current corona device has been mounted to eliminate the charges of the web, allowing the web to deviate from the drum surface. The drum surface is then precharged with the corona device and cleaned from any remaining printing ink particles, after which the surface is ready for a new printing cycle, which may equally well be identical with the preceding cycle as different from this.
As described above, monochrome print can be produced on one side of a paper at one single printing station using black printing ink. In multicolour printing, the different printing inks are applied to the paper at several consecutive printing stations, which operate with different colours, adding the colours one by one to the print generated on the moving web. Double-sided printing of a paper can further be achieved by disposing printing stations as described above on both sides of a moving paper web.
After a print composed of one or more printing inks has been applied to the paper as described above, the print is adhered at a fixing station disposed on the web path. Adhesion takes place by means of infrared radiators, which heat the web surface, resulting in fusion of the polymer printing ink particles to the paper. Eventually, the finished printed web can be either divided into sheets, which are piled or stitched whenever necessary, or it can be rewound.
On principle, similar technique is applied in copying machines and printers, in which the printing substrate consists of individual sheets instead of a continuous web. Besides paper sheets, plastic films are suitable as a substrate in copying machines.
WO patent specification 03/054634 discloses digitally printed papers and boards, whose printing surface consists of a polymer coating containing electrically chargeable ethene acrylate copolymer. The specification examined by means of coronation the chargeability of copolymer of ethene methyl acrylate (EMA), polyethylene terephtalate (PET) and low-density polyethene (LDPE) and also conducted a more comprehensive comparative test series regarding the printing quality obtained in digital printing with boards coated with different polymers. 20% EMA proved the best coating polymer, i.e. EMA in which methyl acrylate monomer accounted for 20 molar %. The results of this specification indicated a markedly lower digital printing quality of low-density polyethene (LDPE) and high-density polyethene (HDPE), which are polyolefins commonly used as the coating of packaging boards.
However, copolymers of ethene acrylate are characterised by being soft and of having a low fusion point, e.g. the fusion point of 20% EMA mentioned above is approx. 80-90° C. Due to their softness, they are exposed to friction and wear when used as the uppermost coating layer on packaging board. Their low fusion point makes them readily heat sealable as such, yet excessively fusionable during sealing, and hence more difficult to control than e.g. the most commonly used heat-sealing polymer LDPE. Due to their stickiness, they also cause problems in extrusion, e.g. by their tendency to adhere to the cooling roll, requiring thus necessarily the adoption of low running speeds.
WO patent specification 03/054634 mentions the stickiness of EMA, which increases as the proportion of methyl acrylate monomer in the polymer increases. The specification has reached an approximate proportion of 15% of methyl acrylate monomer as a compromise between non-stickiness of the coating and high printing quality. The specification also states that it is possible to apply a protective varnish onto the digitally printed surface after fusion of the printing ink, however, this would involve a further work step in the printing process.